Legend of the Far East's Savior Vol. 1 (light novel)

At the end of World War II, a demon-summoning ritual drastically altered the face of the earth. Now, a hundred years later, demons rule the world, and humanity fights back using magicite-powered armored mechs. In the year 2055, a boy named Keita Kawakami enrolls in Japan’s military academy. Possessing memories of an alternate modern era, he successfully pilots a prototype mech that no one else has even been able to activate. When Keita is suddenly deployed to the battlefield, he uses the knowledge from his previous life to achieve results that defy all established norms. Thus begins humanity’s battle to reclaim the world.